1. Field of the Invention
The present invention relates to photovoltaic cells having controllable spectral response.
2. Description of the Prior Art
A photovoltaic cell typically consists of a semiconductor body in which a shallow P-N junction has been produced. When light strikes the device, a current is produced across the junction that is proportional to the intensity of the light source. However, the response characteristics of a photovoltaic cell vary with the wavelength of the incident light. Thus, the cell may produce a certain short-circuit current when illuminated with light at say 600 nanometers (nm), and produce a much higher output current when illuminated with light of the same intensity, but at a wavelength of 900 nm.
The spectral response characteristics of a typical prior art silicon photovoltaic cell are illustrated by the curve 10 of FIG. 1. The response at 900 nm is more than twice that at 500 nm. As described below, these typical spectral characteristics are governed by the silicon band gap structure, and by the light absorption properties of the material. Because of this, in the past it has been difficult to produce photovoltaic cells of altered spectral properties. A principal object of the present invention is o provide a photovoltaic cell having a predetermined, controllable spectral response.
The mechanism by which an output current is produced in a photovoltaic cell involves interaction of the incident light photons with the semiconductor silicon material of the cell. Absorption of a photon by a valence electron in a silicon atom raises that electron to the conduction band and creates a hole-electron pair. In silicon, interaction with a phonon may be required to produce the hole-electron pair as part of a three-particle interaction. The minority carriers (e.g., holes in N-type material) flow to the semiconductor junction, where they are "collected" and add to the output current.
The depth at which the hole-electron pair production occurs depends on the wavelength of the incident light. Specifically, when radiant flux of intensity I.sub.o falls upon the surface of a P-N junction device, it will be absorbed according to the relationship: EQU I = I.sub.o e.sup.- .alpha.x
where I is the intensity at any distance x from the surface, assuming that I.sub.o is the intensity after surface reflection and scattering losses have been substracted from the incident beam. The exponential term .alpha. is the absorption coefficient, which is a function of the cell material and of the wavelength of the incident radiation.
An illustrative curve 11 showing the absorption coefficient .alpha. as a function of wavelength .lambda. for a semiconductor material is shown in FIG. 2. From this it can be seen that the absorption coefficient at the short wavelength end is extremely high. In silicon, at about 350 nm almost all absorption takes place within only a few atoms depth from the surface. Unavoidable surface imperfections in the silicon cause a high order or recombination to occur. That is, as soon as the photon is absorbed to produce a whole-electron pair, recombination of this pair takes place. As a result of this recombination, negligibly few minority carriers reach the cell junction. Output current is very small. This accounts for the low cell output (see FIG. 1) at short wavelengths.
At increasingly longer wavelengths, the photons penetrate further into the silicon body of the cell. The major factor contributing to the spectral response characteristics at such longer wavelengths is the ability of the semiconductor material to translate internal quantum efficiency into external output via charge carrier transport to the P-N junction. In high mobility silicon, a large percentage of the minority carriers produced by photon absorption will be able to diffuse to the junction and add to the output current.
At wavelengths longer than about 900 nm, the minority carrier diffusion length in silicon begins to limit the percentage of minority carriers that reach the P-N junction. As a result, the spectral response of the photocell begins to decline above this value (see FIG. 1). The upper wavelength limit of photocell response is set by the band gap of silicon. Thus photons with an energy of greater than about 1.12 electron volts will be unable to raise the valence electron of a silicon atom up to the conduction band. This photon energy corresponds to a wavelength of about 1100 nm, which is the upper limit of cell response.
For many applications, photocell spectral response other than that shown in FIG. 1 is preferred. Thus, e.g., for some applications a photosensor having a spectral response corresponding to that of the human eye may be required. Human vision has a peak response at about 550 nm, and decreases rapidly above that value to the cut-off wavelength which is below about 700 nm. Photosensors having other spectral response peaks may be needed for special color detection applications such as colorimetry, flame spectroscopy, spectrometry and the like.
In the past, a few techniques have been available to alter selectively the spectral response of a photovoltaic cell. For example, the short wavelength response may be increased by bringing the P-N junction very close to the surface, and by improving the quality of the silicon surface so that is has fewer imperfections.
Another known technique for modifying spectral response is the use of optical filters external to the silicon body itself. Such a filter may comprise a layer or coating of material deposited directly onto the cell surface. A filter film acts to reduce the transmission of radiant flux into the cell at certain wavelengths. While such optical filters do achieve the desired purpose, they have the disadvantage of high cost. Moreover, the coatings tend to wear off and/or to degrade with time, so that over a few months or years the sprectral response of the optical filter cell may vary considerably from its original value. Another object of the present invention is to provide means for selectively altering the spectral response of a photovoltaic cell without the use of an optical filter.